## XPA: Public Access to Data and Algorithms

## Summary

This document is the Table of Contents for XPA.

## Description

The XPA messaging system provides seamless communication between many kinds of Unix programs, including X programs and Tcl/Tk programs. It also provides an easy way for users to communicate with XPA-enabled programs by executing XPA client commands in the shell or by utilizing such commands in scripts. Because XPA works both at the programming level and the shell level, it is a powerful tool for unifying any analysis environment: users and programmers have great flexibility in choosing the best level or levels at which to access XPA services, and client access can be extended or modified easily at any time.

A program becomes an XPA-enabled server by defining named points of public access through which data and commands can be exchanged with other client programs (and users). Using standard TCP sockets as a transport mechanism, XPA supports both single-point and broadcast messaging to and from these servers. It supports direct communication between clients and servers, or indirect communication via an intermediate message bus emulation program. Host-based access control is implemented, as is as the ability to communicate with XPA servers across a network.

XPA implements a layered interface that is designed to be useful both to software developers and to users. The interface consists of a library of XPA client and server routines for use in C/C++ programs and a suite of high-level user programs built on top of these libraries. Using the XPA library, access points can be added to Tcl/Tk programs, Xt programs, or to Unix programs that use the XPA event loop or any event loop based on select(). Client access subroutines can be added to any Tcl/Tk, Xt, or Unix program. Client access also is supported at the command line via a suite of high-level programs.

Choose from the following topics:

- Introduction to XPA
- Access Point Names and Templates
- Getting Common Information About Access Points
- Communication Methods
- Communication Between Hosts
- Distinguishing Users
- XPA User Programs
  - xpaget: get data and info
  - xpaset: send data and info
  - xpainfo: send info alert
  - xpaaccess: get access point info
  - xpamb: message bus emulation
  - xpans: the XPA name server
- XPA Server Routines
  - XPANew: define a new access point
  - XPACmdNew: define a new command access point
  - XPACmdAdd: add a command
  - XPACmdDel: delete a command
  - XPAInfoNew: define an info access point
  - XPAFree: free an access point
  - XPAMainLoop: event loop for select server
  - XPAPoll: poll for XPA events
  - XPACleanup: release reserved XPA memory
  - XPA Server Macros: accessing structure internals
  - XPA Race Conditions: how to avoid them
  - XPA Out of Memory (OOM) errors
- XPA Client Routines
  - XPAOpen: open a persistent client connection
  - XPAClose: close persistent client connection
  - XPAGet: get data
  - XPASet: send data or commands
  - XPAInfo: send an info alert
  - XPAGetFd: get data and write to an fd
  - XPASetFd: read data from and fd and send
  - XPANSLookup: look up an access point
  - XPAAccess: get access info
  - The XPA/Xt Interface: Xt interface to XPA
  - The XPA/Tcl Interface: Tcl interface to XPA
- Tailoring the XPA Environment
  - Environment Variables
  - Access Control
- Miscellaneous
  - XPA ChangeLog
  - Where to Find Example/Test Code
  - User Changes Between XPA 1.0 and 2.0
  - API Changes Between XPA 1.0 and 2.0
  - What Does XPA Stand For, Anyway?

##### Last updated: September 10, 2003